Paramithiotti, Luciano Paolo S wrote:
http://www.timeok.it/files/5_to10_mhz_advanced_doubler.pdf
This design appears to have gone somewhat astray.
high impedance unless of course the transistors enter saturation in which case
the phase noise performance will be severely degraded.
The best place for a balance adjustment circuit is actually in the emitter
circuit.
*The collector balancing work correctly and is more simple to implement.
I contend that the collector balancing technique you use only works
because the doubler isn't operating correctly.
With a high impedance collector output it would be relatively
ineffective unless the balancing resistance is increased to a level that
degrades the phase noise performance or saturation occurs.
The description of the biasing is misleading in that the actual bias level that
sets the crossover current is determined by the signal dependent voltage>across
the two 0.1uF capacitors in the emitter.
With a 1:1 input transformer the quoted figure of 35 ohms for the input impedance
seems excessive for large signal operation of the CB stages unless of>course
they saturate.
*the input impedance is 35 Ohms @ 0dBm as measured with network vector
analyzer. It can be upgraded to 50 ohms adding resistance on emitters, with
some gain reduction and probably less phase noise. I will do some modification
in the next future, including an input 6 Mhz low pass filter. As you know, the
input signal have to be pure sinewave to avoid unsymmetrical positive and
negative half wave and obvious unbalaced output and high harmonics contens. I
will test also the common emitter configuration to better isolate the doubler
from the input impedance and level variations. Regarding the input level I have
setup it's range, as my personal standard,from +7 to +13 dBm.
I thought as much, the large signal input impedance (this is far more
important than the small signal value) will be much lower.
Since the bias shifts with input signal level the small signal input
impedance that you measured is of little value.
It would also appear that the 20MHz tank 5.6uH + 12pF as drawn is inappropriate
in that it inevitably leads to saturated operation.
A series resonant 20MHz tank from the collector node to ground would be a
better choice.
* The LC on collector is to adapt the impedance between the doubler and the
filter and to cut the higher harmonics. The filter itself contain trap for 15
20 and 30 Mhz.
Maybe so, but the filter input topology adopted is inappropriate for low
phase noise and avoiding saturation.
Attempting to match the (poorly predictable and varying - with
temperature and input signal level) collector output impedance to the
filter input impedance is misguided, just treat the output as a high
impedance source. The 4:1 (impedance ratio) output transformer should
suffice, if necessary you can add a 200 ohm resistor in shunt from the
collector node to Vcc if you need a 50 ohm output impedance. In practice
it may be better to buffer the output with a series transformer feedback
stage with well defined output impedance. Series resonant LC traps from
the doubler collector node to ground should be more effective than
parallel resonant series traps in that the high frequency component
amplitudes at the doubler collector will be significantly reduced rather
than enhanced by the filter.
A snapshot or even a sketch of the collector voltage waveforms would be useful
in showing that the transistors saturate or not.
*Actually the prototype is gone to friend's home and I cannot do any more
measure on it. My next prototype's pubblication will be complete of collector
voltage waveform to better understand the working condition of the doubler
stage. I think the 2N3904 is not the best solution, i will test some more
devices and bias point.
At 10MHz you will find that most wideband transistors will be noisier.
However using transistors with a lower base spreading resistance than
the 2N3904 may be useful.
Thank you
Luciano
note: I'm not a genius, I just try to enjoy myself. If someone follow me, is at
his own risk.
Luciano P. S. Paramithiotti
Bruce
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